LeetCode Meditations: Insert Interval

The description for Insert Interval is quite explanatory:

You are given an array of non-overlapping intervals intervals where intervals[i] = [start_i, end_i] represent the start and the end of the ith interval and intervals is sorted in ascending order by start_i. You are also given an interval newInterval = [start, end] that represents the start and end of another interval.

Insert newInterval into intervals such that intervals is still sorted in ascending order by start_i and intervals still does not have any overlapping intervals (merge overlapping intervals if necessary).

Return intervals after the insertion.

Note that you don't need to modify intervals in-place. You can make a new array and return it.

For example:

Input: intervals = [[1, 3], [6, 9]], newInterval = [2, 5]
Output: [[1, 5], [6, 9]]

Or:

Input: intervals = [[1, 2], [3, 5], [6, 7], [8, 10], [12, 16]], newInterval = [4, 8]
Output: [[1, 2], [3, 10], [12, 16]]

Explanation: Because the new interval [4, 8] overlaps with [3, 5], [6, 7], [8, 10].

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We can start with creating a result array to hold, well, the result:

let result = [];

Then, going through all the intervals, we need to check if we are to put our new interval before or after the current interval, or, if they are overlapping and therefore need to be merged.

As we have seen in the chapter introduction, two intervals don't overlap if the start of one is strictly larger than the other's end, or, if the end of one is strictly less than the other's start.

When those both cases are false, they overlap.

First, we can check whether newInterval comes before interval. In fact, if we first check for this (the "earliest" position we can find to place newInterval), we can return immediately with our newly constructed result.
This is also the greedy approach.

for (let i = 0; i < intervals.length; i++) {
  const interval = intervals[i];

  // newInterval is before interval
  if (newInterval[1] < interval[0]) {
    result.push(newInterval);
    return [...result, ...intervals.slice(i)];
  }
  /* ... */  
}

However, if newInterval comes after the current interval we are looking at, we can just push the current interval to our result:

for (let i = 0; i < intervals.length; i++) {
  /* ... */
  // newInterval is after interval
  else if (newInterval[0] > interval[1]) {
    result.push(interval);
  }
}

The last option is when they overlap, in that case, we need to merge the two intervals. We can create newInterval again with the minimum value of the intervals as the start, and the maximum value of them as the end of the new interval:

for (let i = 0; i < intervals.length; i++) {
  /* ... */
  // overlapping, create newInterval
  else {
    newInterval = [
      Math.min(newInterval[0], interval[0]),
      Math.max(newInterval[1], interval[1])
    ];
  }
}

Our loop currently looks like this:

for (let i = 0; i < intervals.length; i++) {
  const interval = intervals[i];

  // newInterval is before interval
  if (newInterval[1] < interval[0]) {
    result.push(newInterval);
    return [...result, ...intervals.slice(i)] 

  // newInterval is after interval
  } else if (newInterval[0] > interval[1]) {
    result.push(interval);

  // overlapping, create newInterval
  } else {
    newInterval = [Math.min(newInterval[0], interval[0]), Math.max(newInterval[1], interval[1])];
  }
}

We also need to push the latest newInterval that we created. And, at the end, we can just return the result:

function insert(intervals: number[][], newInterval: number[]): number[][] {
  /* ... */
  result.push(newInterval);
  return result;
}

Finally, the solution looks like this:

Input: intervals = [[1, 3], [6, 9]], newInterval = [2, 5]
Output: [[1, 5], [6, 9]]

Time and space complexity

The time complexity will be $O(n)$O(n) as we do constant operations for each item in the intervals array. The space complexity will be $O(n)$O(n) as well as we keep a result array and its size will increase as the length of intervals increases.

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Next up, we'll take a look at Merge Intervals. Until then, happy coding.

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